The processes for conversion of petroleum feedstock require large quantities of hydrogen. Currently, this hydrogen is mainly produced by the SMR (Steam Methane Reforming) process, which generates large amounts of CO2, the main greenhouse gas.
The method commonly used for hydrogen production is therefore the SMR process. In the SMR process, the heat required for the steam reforming reaction is supplied by combustion, which takes place in the furnace (in the presence of air). At the outlet of this furnace, the emissions of CO2 are therefore necessarily diluted.
Another method is also used: this is autothermal reforming. In this case, the heat required for the steam reforming reaction is supplied in the reactor by combustion of some of the methane by injecting pure oxygen or air. In this method, the CO2 must then be separated from the synthesis gas if we wish to produce hydrogen. Moreover, this method requires large quantities of oxygen.
The use of an O2 carrier in methods employing circulation of solids for producing synthesis gases has also been proposed. Such a process is shown schematically in FIG. 1. These circulating bed processes are difficult to implement, especially at high pressure. Moreover, the reaction is not controlled in the so-called “fuel-oil” reactor, where the main reaction is the reaction of partial oxidation of the fuel.
Document WO-A-96/33794 describes a method using a fixed bed reactor exposed successively to a reducing gas such as methane and to an oxygen-containing gas such as air. The catalyst is selected from the pairs Ag/AgO, Cu/CuO, Fe/FeO, Co/CoO, W/WO, Mn/MnO, Mo/MoO, SrSO4/SrS, BaSO4/BaS and mixtures thereof. Heat transfer by the oxygen carrier is mentioned, and the heat released during oxidation of the metal can thus be used for endothermic reactions such as oxidation of the reducing gas. The hydrogen produced during the step of reduction of the solid is diluted with the other products of the reactions taking place in the reactor and notably CO, CO2 and H2O. To remove the CO2, it is proposed to add, downstream of the reactor, a second reactor in series containing CaO that can react with the CO2 to form CaCO3, which can be regenerated to CaO during the oxidation step. The CO2 is then diluted with the depleted air.
Document WO-A-99/11591 describes a method of autothermal methane reforming, employing 2 interconnected fluidized bed reactors, with a metal oxide circulating between them. The heat generated during the reaction of oxidation of the metal is used in the second reactor for the reaction of partial oxidation of methane: CH4+2MOX+1→H2+CO+2MOX+H2O as well as the steam reforming reaction: CH4+H2O→CO+2H2 in the case when steam is co-injected with the methane into the reactor. For producing hydrogen at high pressure, both of the 2 interconnected reactors must be at the same pressure, which means compressing the air at the inlet of the first reactor. To avoid excessive energy consumption, it is proposed to integrate this system in co-generation plant equipped with a gas turbine, where a proportion of the air flow compressed by the turbine is diverted to the metal oxidizing reactor.
Document WO-A-2008/036902 describes a method similar to the preceding method, employing a fixed bed reactor submitted successively to an oxidation step and then a reduction step with the aim of converting a hydrocarbon to a gas mixture containing CO+H2. The solid used here corresponds to one of the two formulae: (a) CexByB′zB″Oδ, where B=Ba, Sr, Ca, or Zr; B′=Mn, Co, or Fe; B″=Cu; 0.01<x<0.99; 0<y<0.6; 0<z<0.5; and 1<δ<2.2; (b) SrvLawBxB′yB″zOδ, where B=Co or Fe; B′=Al or Ga; B″=Cu; 0.01<v<1.4; 0.1<w<1.6; 0.1<x<1.9; 0.1<y<0.9; 0<z<2.2; and 3<δ<5.5). The document also proposes a variant employing circulating fluidized bed reactors.
Document U.S. Pat. No. 6,797,253 proposes an adaptation of the method described in document WO-A-96/33794 so as to be able to convert gases containing high concentrations of H2S. The method described employs a fixed bed reactor, which undergoes several successive steps during which the reaction fronts progress along the reactor. During the reforming step, sulphur is fixed on nickel, forming NiS, and the nickel is regenerated during the regeneration step by oxidation with air, producing NiO+SO2.
Document WO-A-00/00427 describes a method producing synthesis gas from methane and using a bed of metal oxide undergoing a reduction step and an oxidation step in succession. At the beginning of the cycle of reduction of the solid, CO+H2 is not produced immediately, therefore it is proposed in this document to recycle the gases produced initially to the inlet of the reactor mixed with methane, until CO+H2 is obtained in the outgoing gas mixture.
However, in all the methods described above, the CO2 produced is diluted and it is therefore necessary to carry out expensive separation steps for recovering the CO2.
The invention aims to remedy this drawback by providing a method for preparing hydrogen (or synthesis gas) with intrinsic capture of CO2 by producing effluents with high CO2 concentration.